The present invention relates generally to the field of charged particle beam technology and particularly to a method of selectively depositing a charged particle beam on a sample based on the charging characteristics of a sample in a charged particle beam imaging device such as a scanning electron microscope.
Particle beam imaging devices, such as a scanning electron microscope (SEM) are well known. Unlike optical microscopes, a SEM uses an electron beam rather than a light beam to obtain an image of a sample. The short wavelength of the electron beam allows SEMs to achieve resolutions that are not possible with traditional optical microscopes. Hence, SEMs are often used to inspect objects having very small dimensions, such as semiconductor devices with intricate patterns.
While a SEM can achieve greater resolution than an optical microscope, a SEM suffers from problems associated with charging artifacts, a phenomena not associated with optical microscopes. SEMs rely on the observation of electron signals from an object being analyzed to obtain information about the topography and composition of the object. Charging artifacts greatly impede this process. A SEM relies on the detection of secondary electron emissions from an object onto which an electron beam is directed, the level of secondary emission giving an indication of the surface topography of the object. An example of what is known as a charging artifact is when a portion of an object collects and stores electrons, i.e.; collects charge. The stored charged (charging artifact) affects the level of secondary electrons, which then is no longer a good indication of the object topography. Charging artifacts have been discussed extensively in many scientific journals and papers, as well as in patent documents such as U.S. Pat. Nos. 3,919,553 and 5,302,828. Therefore, they are a well known phenomenon to those skilled in the art.
There are many types of charging artifacts that can be linked to a multitude of imaging problems. Although some of these phenomena have been extensively studied, no integrated platform for the comprehensive solution of the charging problem has been proposed or implemented. One particular charging artifact that is relevant to the present invention is one which may be called partial charging. This phenomenon occurs when a specimen being viewed contains features that are electrically conductive, e.g. metals, as well as features that are non-conductive, e.g. pure silicon. The conductive features dissipate charge easily while the nonconductive features tend to collect the charge. Hence, the charge artifacts are found only on the non-conducting features. For specimens having this type of topography, it would be useful to determine the areas that are prone to charging versus those areas which are not. Having such a map would allow a system to selectively deposit the electron beam on a particular feature based on the charging characteristic. The prior art does not provide an effective way to accurately map the charging characteristics of a specimen, and as a result, selective deposition is currently not employed.
It is therefore an object of the present invention to characterize or monitor the charging state of a sample so that the charging areas can be identified and mapped.
It is another object of the present invention to scan the electron beam in such a way that charging artifacts are equalized or eliminated in a sample.
The present invention provides a method of preparation of a map of areas on a sample that collects charge, and a method for using the map to selectively scan and modulate the intensity of the electron beam of a SEM so as to discriminate between the charging and non-charging areas of the sample. The present invention is able to reduce contrast artifacts in charging samples and facilitate the restoration of the dynamic range of the image so that sample features can be more easily interpreted.
It is generally known that conventional raster scanning of insulators under certain conditions introduces charging artifacts due to the relatively high intensity of the electron dose. Raster scanning also does not take into account the charge state of the points being irradiated. The present invention provides a method of scanning the electron beam or other type of particle beam so as to take into account the charge condition of the sample and equalize the charging effects.
To generate the charging map, an image is first checked for saturation. The frame for the image is acquired by using digital scan control coupled with digital acquisition of the secondary electron detector signal. The next step is to perform a xe2x80x9cfast scanxe2x80x9d where the first frame is taken at the maximum frame rate that the system is capable of, which is about 1.5 seconds per frame in the current implementation. A fast scan does not allow time for significant charge to collect on surfaces, and this provides a base level to subtract from a slower scan that allows charge to accumulate. Areas where the difference between the two is larger indicate areas of charge collection. A xe2x80x9cslow scanxe2x80x9d is then performed where the frame is acquired in about 23 seconds. The frames are then subtracted pixel-by-pixel in order to isolate the charging component of the image. A lower bound on the pixel intensity is used as the threshold in the subtraction to reduce noise effects. After the pixel-by-pixel subtraction, the charging map is created.
The charging map derived using only the steps described above is not in an optimal form for beam pulsing because the charging pixels are too sporadic and discontinuous, resulting in an excessive noise level. To obtain a more ideal charging map, further image processing is performed to reduce the noise level as well as to merge pixels together to form a fuller representation of a charging feature. This form of the charging map is then optimal for use in the selective deposition method for charging reduction according to the present invention.
The selective deposition process for charging reduction is accomplished by modulating the electron beam intensity to adjust the dosage on a sample based on the charging map. The process begins by generating the charging map as described above. The total charge build-up on the charging areas is controlled by depositing the beam on the charging areas only on selected scans. In the next step, the duty cycle for the charging area is set by determining an interleave factor x defined as the alternate number of scans during which the charging area is exposed to the electron beam during a series of scans of the sample. The non-charging areas are preferably exposed to the beam during every scan, which, together with averaging performed using a plurality of scans, maximizes the S/N (signal-to-noise) ratio. The entire process is repeated until the selected number of frames have been exposed. The repeated steps are the charging map generation and charging reduction.
The mapping and selective deposition processes are preferably controlled by software running on a PC coupled to the SEM. Also implemented is a frame by frame capture function to store the results. The frame capture and averaging are done using image buffers stored in the PC memory to eliminate the need for unnecessary scans of the sample.